The high strength steel which is used in large amounts for machines, automobiles, bridges, and building structures is medium carbon steel with an amount of C of 0.20 to 0.35%, for example, SCr, SCM, etc. defined by JIS G 4104 and JIS G 4105 which is quenched and tempered. However, in all types of steels, if the tensile strength exceeds 1300 MPa, the risk of delayed fracture occurring becomes larger.
As methods for improving the delayed fracture resistance of high strength steel, the method of making the steel structure a bainite structure or the method of refining the prior austenite grains is effective.
PLT 1 discloses steel which is refined in prior austenite grains and improved in delayed fracture resistance, while PLT's 2 and 3 disclose steels which suppress segregation of steel ingredients to improve the delayed fracture resistance. However, with refinement of prior austenite grains or with suppression of segregation of ingredients, it is difficult to greatly improve the delayed fracture resistance.
A bainite structure contributes to improvement of the delayed fracture resistance, but formation of a bainite structure requires suitable additive elements or heat treatment, so the cost of the steel rises.
PLT's 4 to 6 disclose wire rods for high strength bolts containing 0.5 to 1.0 mass % of C in which an area ratio 80% or more of the pearlite structure is strongly drawn to impart 1200N/mm2 or more strength and excellent delayed fracture resistance. However, the wire rods which are described in PLT's 4 to 6 are high in cost due to the drawing process. Further, manufacture of thick wire rods is difficult.
PLT7 discloses a coil spring in which development of a delayed fracture after cold-coiling is prevented, using an oil tempered wire having a hardness in the inner part of cross section of ≧Hv 550. However, the coil spring has a surface layer hardness after nitriding of Hv 900 or more, and a product, for example, in the form of bolt or PC steel bar has a low delayer fracture under a high load stress. Thus, developing a delayed fracture in a severe corrosion environment is a problem.
PLT8 discloses a high strength steel having excellent delayed fracture resistance mainly comprised of tempered martensite structure, which is obtained by nitriding a steel having a certain composition. The high strength steel disclosed in PLT8 displays a delayed fracture resistance even in a corrosion environment containing hydrogen.
Nevertheless, corrosion environments have recently become severe, and a high strength steel displaying excellent delayed fracture resistance even in severe corrosion environments is needed.